1. Field of the Invention
This invention relates to compositions and methods for coating metal substrates. In particular, this invention relates to compositions and methods for forming a polymeric coating on a metal surface by dip autopolymerization. Importantly, the polymerization requires no application of external driving force, such as thermal or electrical energy. The coatings thus formed are uniform, conformal and pinhole-free. The method is suitable for forming coatings with a variety of desirable properties, such as corrosion and erosion resistance, abrasion resistance, and electrical and thermal insulation.
2. Brief Description of the Prior Art
Surface treatment, i.e., coating, of metals plays an important role in metal applications. Such coating processes may be applied for protective or decorative purposes. Conventional coating processes include, for example, spray painting, electrode position, electrophoresis, powder coating, anodization, and chromate conversion coatings. Metals ordinarily treated by these methods include iron, aluminum, copper, and their alloys. Each of these process technologies are currently used extensively in the metal finishing industry, and each has certain advantages and disadvantages.
For example, U.S. Pat. Nos. 5,232,560 and 5,466,357 to Bell et al. describe a method for electropolymerization in a substantially aqueous solution to form copolymer coatings onto electrically conductive substrates. Similarly, as described in U.S. Pat. No. 5,238,542 to Bell et al., cyclic N-substituted methacrylamide monomers may be electropolymerized onto electrically conductive filler. Electropolymerization is advantageous in that it allows the deposition of thick, thermally stable coatings. But while suitable for their intended purposes, electropolymerization requires the application of electrical energy as a driving force.
The practice of the other above-mentioned methods can be expensive, and result in undesirable waste products that must be subsequently disposed of. Spray painting may require a separate prepolymerization step prior to application of the coating. Electrode position and anodizaton require specialized equipment and thus capital investment. Many technologies require the input of heat or thermal energy as a driving force for the deposition processes. For these reasons, industry is in constant search of less expensive technologies to improve competitiveness in the world marketplace. In addition, increasing environmental concerns call for cleaner technologies to meet government regulations and reduce waste treatment and disposal cost.
In particular, aluminum is the most widely used metal today by volume. Areas of application for aluminum range from aerospace to marine to architectural. In almost all cases where aluminum is used, a surface treatment and finishing process are applied. The two most common surface treatments for aluminum is anodization and conversion coating. Anodization is an energy-intensive process, using phosphates. Conversion coating uses chromates and other heavy metal ions. Both of these treatments thus generate environmentally hazardous waste, which must be properly disposed of. Accordingly, there remains a need for efficient, inexpensive compositions and coating methods for aluminum and other metals that generate minimal waste.
The above-discussed and other problems and deficiencies of the prior art are overcome or alleviated by the composition and method of the present invention for coating metals by dip autopolymerization, wherein organic monomer in an acidic solution undergoes autopolymerization upon contact with a metal substrate at room temperature, thereby forming a polymeric coating on the metal substrate. Importantly, the polymerization requires no application of external driving force, such as thermal or electrical energy. The coatings thus formed may be up to 50 or more microns thick and have molecular weights up to about 250,000 or more.
In one preferred embodiment of the present invention, the composition comprises an acidic solution of an organic electron acceptor monomer that undergoes autopolymerization in contact with a metal substrate, thereby forming a polymeric coating on the substrate. In another preferred embodiment of the present invention, the composition comprises an acidic solution of an organic electron acceptor monomer and an organic electron donor monomer that undergo autopolymerization in contact with a metal substrate, thereby forming a polymeric coating on the substrate. By electron acceptor monomer it is meant a monomer or small polymer having at least one electron withdrawing group, capable of further polymerization with the electron donor monomer. By electron donor monomer it is meant a monomer or small polymer having at least one electron donating group, capable of further polymerization with an electron acceptor monomer. Other metals suitable for use in the practice of the present invention include copper, iron, and zinc.
In another embodiment of the present invention, there is described metal-polymer composites comprising a metal such as copper, aluminum, iron, zinc, steel, or alloys thereof, and a polymeric coating.
In accordance with the method of the present invention, an acidic monomer solution is provided, and a clean metal substrate is submerged (dipped) into the monomer solution for a prescribed period of time. Polymerization occurs on the metal surface in the monomer solution spontaneously without application of any external driving force, such as thermal or electrical energy. Rather, polymerization is initiated at the metal-solution interface due to the interaction of the metal surface with the monomer in solution. The monomer converts directly to a polymer coating on the metal surface and thus obviates the need for the prepolymerization step sometimes required in conventional coating methods.
The present invention results in the formation of a uniform, conformal and pin-hole free polymeric organic coatings on metal surfaces. Physical and chemical properties such as corrosion and erosion resistance, electrical or thermal insulation, adhesion, and scratch resistance of the coating may be varied simply by varying the composition of the monomer solution. The polymer coatings formed may also be used as a primer for further surface finishing treatments or as a final coating.
The process can be very easily applied in various industrial settings. Relative to other polymerization techniques, the method is robust, being insensitive to moisture and other impurities. No special metal treatment is required other than cleaning of the metal surface. In the practice of the method, the low cost of operation relative to other coating processes will result in considerable energy savings. Sophisticated equipment is not required, so capital equipment cost is also minimal. Solvent use is-minimal, and no heavy metals-are required to be present. Also, water is frequently used a co-solvent with a suitable solvent to dissolve the monomer, resulting in a more environmentally acceptable coating composition and method.